Touch screens are commonly used in electronic devices such as computers, laptops, desktops, tablets, televisions, and cellular phones, to enable a user to control the devices by moving the user's finger or a stylus near or on the touch screen. For this purpose, a touch screen may be superposed with a transparent capacitive sensor matrix which senses the user's finger or stylus and generates data relating to the position of the object. This data is analyzed by a monitoring module and is fed to the electronic device, where a cursor could be created for tracking the object's motion, and certain gestures by the user are converted to instructions (for example, for manipulating an object). Capacitive sensors are usually divided into mutual-capacitance sensors and self-capacitance sensors.
Mutual capacitance is capacitance that occurs between two charge-holding objects or conductors, in which the current passing through one passes over into the other. The current between the two objects decreases when a third object is brought toward the first two objects. A mutual-capacitive sensor uses this fact to sense when and where a user's finger or conductive stylus touches the sensor. A mutual-capacitive sensor known in the art generally includes a grid formed by two perpendicular groups of strips (lines) of conductive material. Each group is composed by a respective number of parallel lines. A capacitor is present at each intersection between lines. A voltage is applied in a sequential fashion to the vertical lines of the grid, and an output voltage is measured on the horizontal (measurement) lines. Bringing a finger or conductive stylus near the surface of the sensor changes the local electric field which reduces the mutual capacitance effect. The capacitance change at every individual intersection on the grid can be measured to accurately determine the touch location by measuring the voltage in the rows of the grid. Mutual capacitance allows multi-touch operation where multiple fingers, palms or styli can be accurately tracked at the same time.
Self-capacitance requires only one electrode which holds a “floating capacitance”. The floating capacitance is influenced by parasitic capacitance between the electrode and surrounding electric conductors. Since the human body is a conductor, when a finger is placed close to the electrode, the value of the floating capacitance increases and can thereby be detected through a measurement terminal. Self-capacitance sensors can have the same X-Y grid as mutual capacitance sensors, but the columns and rows operate independently. With self-capacitance, a voltage drop is indicative of the capacitive load of a finger on each column or row located near the finger. This produces a stronger signal than mutual capacitance sensing, but the X-Y grid self-capacitance sensor is unable to resolve accurately more than one finger or conductive stylus at a given time.
Another type of a self-capacitance sensor is described in patent publications WO 2010/084498 and US 2011/0279397, which share the inventors and the assignee of the present patent application. In the self-capacitance sensor described in the above-mentioned applications, a two dimensional array (a matrix) of pads is provided to sense the change in the electric field induced by a finger or conductive stylus on each pad, thus enabling multi-touch operation.